关于自定义类型，返回group元素的定位问题echarts custom配置项内容和展示

自定义类型返回group元素时，对group经行定位，定位失效，这是问什么，求解释！！！

      var data = [
    [36, 36, 'yiyiyi'],
    [72, 160, 'ererer']
]

var renderItem = function(param, api) {
    var children = [];
    children.push({
        type: 'text',
        style: {
            text: api.value(2),
            // x: api.value(0),
            // y: api.value(1),
            font: '12px STHeiti, sans-serif',
            fill: 'black',
            textAlign: 'center'
        }
    });

    return {
        type: 'group',
        left: api.value(0),
        top: api.value(1),
        children: children
    }
}

option = {
    grid: {
        top: 0,
        left: 0
    },
    xAxis: {
        type: 'value',
        show: false
    },
    yAxis: {
        type: 'value',
        show: false,
        inverse: true
    },
    series: [{
        type: 'custom',
        data: data,
        renderItem: renderItem
    }]
};